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Webcast-Catalyst9k
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Will the router add the routes received from other routers using classful protocols?

Dear friends

I came up with a scenario and it actually confused me.

I have configured a packet tracer sample network using two routers and its interfaces configured like this

Interface                         Router-A                                        Router-B

----------------------------------------------------------------------------------------------------

s0/0                              10.1.0.1/8                                             10.1.0.2/8

fa0/0                             11.11.0.1/16                                       11.14.0.1/24

fa0/1                             11.12.0.1/16                                       not configured

fa1/0                              11.13.0.1/25                                        12.0.0.1/25

Router-A configuration

router rip

network 10.0.0.0

network 11.0.0.0

Router-B configuration

router rip

network 10.0.0.0

network 11.0.0.0

network 12.0.0.0

Router-A does not show any RIP entries using SHOW IP ROUTE commands.

Will the router accept RIP updates from router-B regarding network 11 since it already has interface in it.

Secondly

if I turn all interfaces in Router-A with 11.0.0.0/16 down

then router is ready to receive update from Router-B regarding network 11.0.0.0 of Router-B but it is showing in my

SHOW IP ROUTE command

as 11.0.0.0/25 is variably subnetted

why it is showing /25. Is it for fa1/0 which is configured with /25 mask despite I have turned the interface down.

My main concern is whether Router-A will update the rip update from router-B considering

1. Since it already has interface configured with 11.0.0.0 network (Will it accept rip update or any update for this class of network)

2. Or will it ignore the update totally.

We know theory like

Routers receiving a RIP V1 or IGRP routing advertisement use two methods for deriving the correct mask to associate in the

advertisement, as follows:

1. Advertisements for subnets that are part of the same major network as the IP address of the interface they are received

over will have the subnet mask of the receiving interface applied to them.

2. All other networks in the advertisement should be for major networks, not subnets. This is because the router sending

the advertisement should have summarized the subnets down to the major network before sending the advertisement.

Therefore, the masks for all other networks should be the default mask for the class of address received.

Is the subnet applied based on what already it has or the default or the updates are ignored since router already has interface of that class applied in one of its interface.

I am attaching the packet tracer file with this discussion. Please see the file if required

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Will the router add the routes received from other routers using

Part of what is going on here is that router B will send a summary route for network 11.0.0.0/8. As long as router A is aware of any network 11 routes in its own table then the summary route from router B is less specific and will not be put into the routing table.

HTH

Rick

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